1. Field of the Invention
The present invention relates to a method of picking up a sectional image (i.e., light intensity distribution in a certain cross section). The method of the present invention is suitable especially for measurement of a two-dimensional intensity distribution of laser light for use in laser-annealing a semiconductor thin film.
2. Description of the Related Art
In flat panel type display devices such as an active matrix type liquid crystal device and an organic EL display device, a large number of thin film transistors (hereinafter abbreviated as TFT) are formed on an insulating substrate of glass, plastic or the like in order to individually drive each pixel. As to an amorphous silicon (a-Si) film, since a forming temperature is low, the film can be comparatively easily formed by a vapor phase process, and mass productivity is also superior, the film has been broadly used as a semiconductor thin film for forming a source, drain, or channel region of the TFT.
However, the amorphous silicon film has a disadvantage that the film is inferior to a polycrystalline silicon (poly-Si) film in physical properties such as conductivity (mobility of a-Si is lower than that of poly-Si by two or more digits). Therefore, there is a demand that a technique for forming the source, drain, or channel region of the TFT in the polycrystalline silicon film be established in order to increase an operation speed of the TFT.
For example, an annealing method using excimer laser (excimer laser annealing: hereinafter referred to as an ELA process) is used as a method of forming the polycrystalline silicon film. This annealing method can be performed in a temperature range (i.e., from room temperature to about 500° C.) in which a general-purpose glass substrate is usable.
In the ELA process, for example, after depositing an amorphous silicon film in a predetermined thickness (e.g., about 50 nm) on a substrate, the amorphous silicon film is irradiated with krypton fluoride (KrF) excimer laser light (wavelength of 248 nm), xenon chloride (XeCl) excimer laser light (wavelength of 308 nm) or the like. The amorphous silicon film is locally molten, recrystallized, and changed to the polycrystalline silicon film.
It is to be noted that the ELA process is applicable to various other processes, when an average intensity (fluence) of the laser light is changed. For example, when the intensity of the laser light is set to a range having an only heating function, the process is usable in an impurity activation step for the TFT. When the intensity of the laser light is excessively increased, a rapid temperature rise is caused, and therefore the process is usable for removing the film from the TFT. It is to be noted that the use of the phenomena is not limited to the TFT, and the phenomena are broadly applicable to a semiconductor manufacturing process.
Additionally, in a case where the TFT is formed of a polycrystalline silicon film in order to increase the operation speed in the flat panel type display device like the liquid crystal display device, the organic EL display device or the like, a large fluctuation is generated in a threshold voltage (Vth) of the TFT, when there is a fluctuation in the number or distribution of a crystal grain boundary included in the channel region of each TFT. This is a cause for lowering operation characteristics of the whole display device. Therefore, there has been a demand for a TFT in which the number of crystal grain boundaries is homogenized as much as possible in each channel region, or crystal grains are grown to be larger than the channel region, and positions of the crystal grains are controlled to thereby remove the crystal grain boundary from each channel region.
The present inventors have developed the laser annealing process for forming a silicon film having a large crystal grain diameter. In the process, an optical device (phase-modulation device) referred to as a “phase shifter” is inserted midway in an optical path irradiated with the laser light, and accordingly a two-dimensional intensity distribution of laser light is adjusted on the amorphous silicon film to grow crystal grains having large grain diameters. Here, the phase shifter is an optical device in which a fine plane pattern is formed of stepped portions comprising grooves or protrusion in a transparent quartz substrate. The phase shifter imparts a phase difference to a part of the laser light passing through the shifter, and produces the two-dimensional intensity distribution of the laser light by diffraction of the laser light and interference of laser light having different phases. When the two-dimensional intensity distribution of the laser light is adjusted in this manner, an appropriate temperature distribution is produced on the substrate to be treated. Accordingly, it is possible to control a position of a single silicon crystal having a large grain diameter of about 2 to 7 microns in forming the crystal.
The following has been found as a result of development of the laser annealing process. That is, as to the laser light with which the amorphous silicon film is irradiated, a pattern of the intensity distribution in a micro region having a submicron level is remarkably important for the controlling of the positions of the crystal grains in order to form the grains having large grain diameters. However, a method of exactly measuring the intensity distribution is very difficult partly because the excimer laser light is invisible light of an ultraviolet region, and the method has not been established yet.
The following methods have heretofore been known as methods for measuring the intensity distribution of the laser light for use in the ELA process.
In one of the methods, the amorphous silicon film is irradiated with laser light, and the intensity distribution of the laser light is evaluated based on changes of physical properties of the film. That is, the amorphous silicon film, which is a process target, is irradiated with the laser light with a light intensity (fluence) having such a threshold value that crystallization is induced. Then, an only portion having a high light intensity forms polycrystalline silicon, and the physical properties partially change. Therefore, after irradiating the amorphous silicon film with the laser light, a tissue of the corresponding portion is observed with a microscope, so that the intensity distribution of the laser light can be estimated. It is to be noted that in the evaluation method based on the changes of the physical properties, it is possible to utilize not only the crystallization of the amorphous silicon film but also the changes of physical or chemical properties of another material (e.g., photoresist).
In another method, a substrate exclusive for use in image pickup is used whose surface is coated with a fluorescent agent (Jpn. Pat. Appln. No. 2004-020104). In this method, the surface of a coating film of the fluorescent agent is aligned with a transverse surface where the intensity distribution of the laser light is to be measured. The above-described substrate for image pickup is held, and the surface of the substrate for image pickup is irradiated with the laser light. In this case, fluorescence emitted from the fluorescent agent is two-dimensionally enlarged using an image forming optics, and a sectional image (light intensity distribution) of the laser light is picked up on the back surface of the substrate for image pickup.
Here, the former method of measuring the laser light intensity distribution based on the changes of the physical properties has had the following problem. That is, since the changes of the physical properties depend on a threshold value of the light intensity, the laser light intensity has to be changed in a stepwise manner, and a plurality of times of laser irradiation and physical property evaluation are required. As a result, fluctuations of conditions for the laser irradiation are included in the evaluation results. Since there are also fluctuations in a material itself that causes the physical property changes, it is very difficult to grasp the light intensity distribution as a “plane image”, and the evaluation cannot be said to be correct. Furthermore, since the physical properties are evaluated by offline inspection, much time is required until the results are obtained.
On the other hand, the latter method of picking up the sectional image of the laser light by conversion into the fluorescence has had the following problem. That is, since the fluorescence is visible light, a spatial resolution of the resultant sectional image is not approximately 0.5 μm or less corresponding to a wavelength of the fluorescence. In general, fluorescent materials are toxic, and largely influence environments.